This invention relates to radiographic apparatus for use in computerized axial tomography. It is particularly advantageous in rotating fan beam type tomographic scanners and will be described with particular reference thereto although it will be appreciated that the invention has broader aspects. For example, the radiation beam dimension changing system and the source radiation intensity measuring system are applicable to virtually all radiographic scanners including traverse and rotate type tomographic scanners. Other aspects of the invention will find varying degrees of applicability to other scanners.
Rotating fan beam tomographic scanners normally comprise a circular area in which a patient or object to be examined is placed. A source of X or gamma radiation is mounted to move in an arc adjacent to the patient circle in order to rotate a beam of radiation at least partially around the patient. An arc of radiation detectors for measuring the intensity of radiation passing through the patient are positioned opposite the patient circle from the source of radiation. A processing means transforms the intensity measurements of the arc of detectors into a visual display of the planar area of the patient scanned.
For safety reasons and to meet U.S. Government regulations, the radiation source must be shielded in a housing or enclosure system which essentially traps all radiation except that needed for the tomographic scan. To define the cross sectional dimensions of the radiation beam needed for the scan, tomographic scanners have used a variety of collimators. Also, a variety of shutters and radiation tube controls have been used to block completely the radiation. Examples of prior art collimators and shutters are shown in U.S. patent application Ser. No. 786,358 of Joseph B. Richey, et al., filed, Apr. 11, 1977, Haupt U.S. Pat. No. 2,542,196, Akaski, et al. U.S. Pat. No. 2,851,610, Hura U.S. Pat. No. 3,829,701, Sperry U.S. Pat. No. 3,384,751, and Martin U.S. Pat. No. 3,275,831.
Excessive radiation can cause two types of image degradation. The first is scattered radiation. Stray radiation which is misdirected may, strike physical parts of the scanning equipment and areas of the patient not under examination. This stray radiation may be reflected one or more times and eventually impinge upon the radiation detectors. Because unknown detected stray radiation travels in an unknown path through objects of unknown attenuation, it creates erroneous data signals and noise. This scatter can reduce the resolution of the image produced on the visual display.
A second type of image degradation from excessive radiation is caused by the "penumbra effect." The radiation beams are emitted from a very small area on a radiation tube anode known as the focal point. Theoretically, this spot can be so small and the bombardment of it with electrons so precise that the beam emitted forms a precise and regular conical pattern of "on focus" radiation. In practice, however, the spot is a larger area than a theoretically optimized point and the tube emits a penumbra or band of so-called "off-focus" radiation from areas around the spot. The penumbra or off-focus radiation is another source of image degradation.
A further cause of image degradation is fluctuations in the intensity of radiation produced by the source of radiation. If the intensity of the source of radiation increases or decreases, the arc of detectors measures a similar increase or decrease. The processing system would erroneously attribute the variation in intensity measured by the detectors to variations in the attenuated radiation. This misreading of the attenuation properties causes inaccuracies in the final visual image produced.
In the past, collimator and filter assemblies have been complex, bulky, and inflexible. Changing the collimator size, or inserting a new filter material into the beam path, was a cumbersome and time-consuming operation. In many cases, manual removal and disassembly or replacement of the structure was required for any change.
This manual task required an extended interruption in the scanning of patients and down time of the machine.
In prior art computerized axial tomographic scanners, it was common to interrupt the scanning procedure to check the reference level of radiation produced by the radiation source. In many scanners, it was necessary to remove the object being examined so that the source of radiation could strike the detectors directly. In others, it was necessary to superimpose a detector into the path of radiation; in yet others, a dummy attenuation material inserted between the source and the detectors. Such methods of checking the radiation intensity were not only disruptive to scanning procedure but only intermittently checked the radiation levels.
Another approach was positioning a radiation detector in a position in which it could view the X-ray tube. The present invention contemplates a new and improved apparatus which overcomes the above problems and others, yet provides a collimator assembly which is simple, easy to operate, and versatile.
In accordance with the present invention, a radiographic scanning apparatus with an adjustable collimator assembly is provided. A plurality of collimation members are assembled for varying at least one cross sectional physical dimension of the beam of radiation. Further provided is a control mechanism for causing the collimation members to vary the at least one dimensions without removing the collimator apparatus.
In accordance with another aspect of the invention, a tomographic scanning apparatus is provided wherein a radiation intensity measuring detector is located adjacent the radiation beam for continuously monitoring its intensity. In a more limited aspect, the radiation intensity measuring detector has dual detection elements mounted closely adjacent the collimator members on opposite sides of the beams to measure continuously the average intensity of radiation across the beam without interferring with the beam.
This invention also includes a radioscopic scanning apparatus wherein the control mechanism includes circuitry for automatically and remotely changing filters and cross sectional dimensions of a collimator. A failsafe shutter assembly for protecting a patient from excessive radiation in an equipment malfunction is also provided.
The collimator assembly of the present invention is easily fitted to and readily usable with conventional radiographic scanning apparatus.
Other benefits and advantages of the present invention will become apparent from the description of the preferred embodiment.